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  1. #1
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    Fast charging Nimh bad?

    According to http://greenbatteries.com/nibafa.html

    Does rapid charging reduce the life of batteries?

    No. So long as it is done using a properly designed, smart charger, most NiMH batteries can be recharged in about an hour without any damage or reduction in their life.

    I have a 2-hour charger from walmart rated at 700 mah. I use it charge batteries from 1600-2500 mah. After the batter is charged the charger automatically shuts off. Is it worth it for me to spend $40-$50 on a more expensive slower charger?

  2. #2
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    Depends on the mAh rating of the batteries. 1C is about the high of the tolerable charging range, i.e. a 1000mAh battery chaged at 1C will take about 1 hour. a 2000mAh battery charged in one hour is 2c which is about twice as fast as recommended, though some batteres its still tolerable. Those 15 minutes chargers are the worst they really kill the batteries fast.
    It all comes down to heat, the faster you charge the hotter the battery will get. And since its a bunch of layers the internal core temp of the battery can be much hotter than the outside.
    I've seen people with those 15 minute chargers report only a few dozen uses before they need replaced. I have older nimh that are 10 years old that I have retired to kid toys now.
    Also remember the cheaper chargers and the faster ones have the issue with missing the charge termination. I had what was a halfway decent charger that I spent $30 on and it turned out it terminated the charge early and only put in about 75% of the capacity. so while it charged slow enough to keep the cell lifespan happy it never fully charged so I never got the best runtime from anything I used.
    Thats another advantage of the quality chargers. So to sum it up, not only can you control the charge current to charge at the afest rate, you can also get the best capacity, as well as testing capacity to find out if your cells are getting old/weak/damaged. I test mine about once a year to determine their capacity to see if they need replaced so for me in just a couple years the $70 charger paid for itsself.

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    Quote Originally Posted by enine View Post
    a 2000mAh battery charged in one hour is 2c which is about twice as fast as recommended, though some batteres its still tolerable.
    Small quip there: a 2000mAh battery charged in 1 hour is still 1C. "1C" is not 1000mA or 1A, it's the capacity of the battery, e.g. 2000mA. Therefore 1 hour is the fastest recommended cell charge time, regardless of the cell's capacity.

    That said, a 700mA charger will charge a 700mAh battery in 1 hour, and a 1400mAh battery in 2 hours, so the second battery will be charged at 0.5C. The same charger has a different "C" value for different batteries, but since generally batteries fit in a small range (1100-2900mAh) this isn't a huge problem.

    The "safest" chargers are closer to 0.1C, but most people don't want to waste 10 hours charging; 4- and 2- hour chargers are a fair compromise.

  4. #4
    cyclepath daredevil's Avatar
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    So clarify this for me please. I have a LaCrosse charger that has the capability to charge from 200 - 1800 depending on the number of batteries being charged. Generally speaking, which charge rate should I use to get the best life of the battery? 200?
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    Quote Originally Posted by daredevil View Post
    So clarify this for me please. I have a LaCrosse charger that has the capability to charge from 200 - 1800 depending on the number of batteries being charged. Generally speaking, which charge rate should I use to get the best life of the battery? 200?
    That all depends on what criteria the charger uses to determine when to terminate the charge and what capacity cells you are talking about.

    Charge rates of about 1/10 C are popular because the charge time is convenient, however in most cases a higher charge rate will allow cleaner determination of the cut off point.

    NIMH are not as good at disipating overcharge as (for example) nicad.

    A faster charge isn't always worse for the cells, often slow chargers are more damaging.

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  6. #6
    cyclepath daredevil's Avatar
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    Quote Originally Posted by Unknown Cyclist View Post
    That all depends on what criteria the charger uses to determine when to terminate the charge and what capacity cells you are talking about.

    Charge rates of about 1/10 C are popular because the charge time is convenient, however in most cases a higher charge rate will allow cleaner determination of the cut off point.

    NIMH are not as good at disipating overcharge as (for example) nicad.

    A faster charge isn't always worse for the cells, often slow chargers are more damaging.

    Thanks UC.
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    From what I've read around, the biggest obstacle for me about a fast charger is that the really fast chargers don't 100% charge the battery. That last 10-20% charge needs time and a low current to get it (apparently, but what people have written about testing capacity after charging with different chargers).

    The source for this is people who bought Lacrosse or Maha charger that will drain the battery and tell you how many millamps it got out of it. A lot of the fast chargers are capable of 100% charging the battery - IF you leave it on the charger overnight where it will get trickle charged...

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    Quote Originally Posted by PaulRivers View Post
    From what I've read around, the biggest obstacle for me about a fast charger is that the really fast chargers don't 100% charge the battery. That last 10-20% charge needs time and a low current to get it (apparently, but what people have written about testing capacity after charging with different chargers).
    I think that's true, but I don't see how it's an 'obstacle' to using a fast charger, especially since many of them automatically switch to a trickle-charge mode after the fast initial charge. If I need to get some cells charged in an hour then I'm happy to get them to 90% charged in my fast charger instead of only 10% charged using a slow charger for an hour. OTOH, if I have more time for the charge then the fast charger will switch to trickle mode and will finish getting them up to 100% charge.

    BTW, the numbers in the OP don't look consistent. If the charger is rated at 700 mA then it'll take about 4 hours to fully charge the 2500 mA-hr cell (2500 mA-hr / 700 mA plus a little extra to allow for inefficiency) and the 1600 mA-hr cell would take almost 3 hours. So it really isn't a 2-hour charger for cells in that capacity range. As long as it is properly sensing when the cells are at full capacity and shutting off at that point then I see no reason to get a different charger - esp. one that's even slower.

  9. #9
    Senior Member rscamp's Avatar
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    The cell construction also determines tolerance to fast charging. Just as for NiCds, NiMHs designed for low impedance/high current will tolerate fast charge much better than those built for high capacity/low current.
    Rob

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